I just got a new home broadband router that has both 2.4GHz & 5GHz
and only then did I realize that both the Windows and Linux laptops
don't have a 5GHz Wi-Fi card (both only have 3.4GHz 802.11 b/g/n).

Neither has either 5GHz or 802.11 ac.
What are my options?

Note: I called Dell Hardware Sales for the "Inspiron 15 3521" at
800-289-3355 but they don't even have a part number for an internal
5GHz NIC, so they suggested an external USB stick, Dell part number:
$60 + tax (free shipping) Netgear A6068352
$50 + tax (free shipping) Linksys A8024912

Googling, I find some of the specs he

"Netgear N900 Wireless Dual Band USB Adapter"
http://accessories.us.dell.com/sna/p...sku=A6068 352

"Linksys Mini AC Adapter AC580"
http://accessories.dell.com/sna/prod...sku=A8024 912

My question is whether any of you experts has experience fitting an
*internal* 5GHz NIC into a Dell laptop, and whether it's true what
Dell said, which is that no internal Wi-Fi card will work?

If I must add 5GHz Wi-Fi externally, do you have suggestions as to how
to get a good 5GHz (ac) Wi-Fi USB stick that is both convenient and
powerful at a good price?